Two independent water balance approaches demonstrate extensive use of groundwater by rainfed crops

In the rainfed cropland of the U.S. Corn Belt effective management requires an understanding of where crop water originates. Thus, quantification of where in the subsurface crops extract water from is warranted. In this study, we co-located measurements of crop water use, soil moisture, and groundwater level at three locations within a field with potentially sustainable management practices (ALT field). The field has a restrictive claypan soil layer previously thought to restrict vertical water movement. We used these measurements to calculate a point-scale water balance and show that crops draw important amounts of water from below the restrictive claypan. These calculations contain considerable uncertainty but show that between 31 % and 53 % of growing season water use is sourced from below the claypan. There was considerable variability between the three locations. To test if our results were robust over larger spatial scales and a longer period, we calculated the groundwater flux with independent field-scale water budgets using measurements of precipitation, evapotranspiration ( ET ), soil moisture, and runoff. We did this at the ALT field as well as a field managed with the prevailing practices of the region (PRV). This field-scale water balance showed that both fields drew considerably from water beneath the claypan, supplying an average of 29 % and 25 % of total growing season ET for the ALT and PRV fields, respectively. Values of non-growing season groundwater flux were not sufficient to recharge this water use, highlighting the importance of complex groundwater recharge pathways in rainfed croplands of the U.S. Corn Belt.